Windows Programming Using C#

Internet Programming

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Contents

Basic Internet classes Internet protocols Client TCP/IP sockets Server TCP/IP sockets UDP sockets

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DNS Class

The DNS class provides a simple interface to the domain name system

This allows you to translate names to IP addresses and vice versa

Remember that a computer with one name might have several IP addresses if it has several network cards

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DNS Class

Methods IPHostEntry GetHostEntry(string host)

Returns an IPHostEntry for the host The host string can be a host name or a dotted

address If an empty string is passed as the host,

information is returned about the local machine

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IPHostEntry Class

This holds all the information on a particular host Properties

IPAddress[] AddressList Returns an array of IP addresses for the host If the host cannot be resolved, this list has zero length

string[] Aliases Returns a list of aliases for the host

string HostName Returns the primary name for the host

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IPAddress Class

This represents an IP address with support for IPV6

Methodsbyte[] GetAddressBytes()

Returns an array of bytes in the address

string ToString() Returns the address in the usual dotted notation

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IPAddress Class

Many useful, pre-defined fields are providedAny

Matches any IP address Used for listening to accept any host

Broadcast An address which sends to all hosts on the local

area network

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IPAddress Class

Loopback The loopback address for the local host

None An address which does not match any real

address

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Communication Modes

An internet connection is one of two typesConnection-oriented

Similar to a phone connection A virtual path is set up between two hosts and the

communication parameters are negotiated Guarantees that the packets are delivered and in

the correct order

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Communication Modes

Connectionless This is like sending a letter There is no permanent connection between sender

and recipient This saves

Setup time Additional information in the packets to ensure they are

received and in order Connectionless protocols do not guarantee

delivery or delivery in the right order

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Protocols

TCP Transmission control protocol Connection-oriented protocol with guaranteed delivery Used for most communication with servers

UDP User datagram protocol A connectionless protocol without guaranteed delivery More efficient than TCP Used for streaming media

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Sockets

The software connection to the internet is called a socket A socket combines

The IP address of a machine A port number

Each program which opens a socket to communicate on the internet uses one of 65,535 sockets on the machine

Data from the network is delivered to a particular socket and then to a particular program listening on that socket

This allows one computer to have many network connections active at once.

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Sockets

A socket can be of different types TCP/IP

Acts as a data stream between two computers which can be read from and written to

The socket is set up to establish a virtual circuit to a single host

UDP Send discrete messages between computers Each message can be sent to a different computer There is no permanent connection between computers

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IPEndPoint Class

Represents one end of a socket connection

IncludesThe IP address of a computerA socket number

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AddressFamily

An enumeration of the types of addressing that can be used by a socket

Common values includeAppleTalk InterNetwork // IPV4 InterNetworkV6 // IPV6

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SocketType

An enumeration of the types of sockets that can be created

Common values Dgram

Connectionless UDP Raw

Access to underlying protocol Used for ICMP

Stream Connection-oriented TCP/IP

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ProtocolFamily

The protocol used by a socketRaw

Raw protocol

Tcp Transmission control protocol

Udp User datagram protocol

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Socket Class

This creates a real socket The namespace is System.Net.Sockets To create a socket

Socket s = new Socket(AddressFamily.InterNetwork,

SocketType.Stream, ProtocolType.Tcp);

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Socket Class

To connect a socketIPHostEntry hostEntry = Dns.GetHostEntry(hostName);

IPAddress[] addresses = hostEntry.AddressList;

IPEndPoint endPoint = new IPEndPoint(addresses[0], port);

s.Connect(endPoint);

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Socket Class

To check to see if the socket is connected If(s.Connected) { … }

To write data to the sockets.Send(byte[], length, offset);

To read from a socketnread = s.Receive(byte[], length, offset);

*see my_curl

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Servers

A server Creates a TcpListener This listens on a particular port for connections When a connection is received, a socket for the

connection is returned Data is exchanged over the socket The socket is closed The listener listens for the next connection

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TcpListener

To create a server

int port = 2001;

IPAddress localAddr = IPAddress.Parse("127.0.0.1");

TcpListener listener = new TcpListener(localAddr, port);

listener.Start();

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TcpListener

To wait for a connectionSocket sock = listener.AcceptSocket();

To communicate on a socketUse the Send and Receive methodsClose the socket when finished

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To Connect to a Server

There are two waysUse a TcpClient

Create a client and connect TcpClient client = new TcpClient(server, port);

Get a stream to read and write to the server NetworkStream stream = client.GetStream();

Read/write the stream stream.Write(data, 0, data.Length); Int32 bytes = stream.Read(data, 0, data.Length);

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To Connect to a Server

Use a Socket Create a normal Socket and connect to the host

and port Use Send and Receive to communicate Close the socket when done

*see hello_client and hello_server

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UDP

This sends discrete messages rather than a stream of bytes

Each message can be sent to a different computer

UDP is much more efficient than TCP/IP UDP is used for sending audio and video

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UdpClient

While you can use a socket for UDP communication, it is easier to use a UdpClient

To create a client and listen on a port UdpClient client = new UdpClient(port);

To send bytes to a computer byte[] sendBytes = Encoding.ASCII.GetBytes(msg); client.Send(sendBytes, sendBytes.Length, host,

remotePort);

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UdpClient

To receive bytes from any computerIPEndPoint remoteIpEndPoint = new IPEndPoint(

IPAddress.Any, 0);Byte[] receiveBytes = client.Receive(ref remoteIpEndPoint);string returnData = Encoding.ASCII.GetString(receiveBytes);

The IPEndPoint passed to Receive is filled in with the address of the sender of the message

To find the message sender remoteIpEndPoint.Address remoteIpEndPoint.Port

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Socket Timeouts

The Receive call blocks until it receives a message

This can be a problem if nobody wants to talk to you

You can only wait for the phone so long… You need to set a timeout which will

interrupt the receive operation

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Socket Timeouts

The time is expressed in millisecondsclient.Client.SetSocketOption(

SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 5000);

When the timeout fires, an exception is thrown, which must be caught

* see UdpChat

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Broadcast

Ever feel like just calling anybody? Then broadcast it! A broadcast message will be picked up by

any listener on the local area network Broadcast messages will not pass through

routers

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Broadcast

To send a broadcast message use IPAddress.BroadcastIPEndPoint remoteIpEnd =

new IPEndPoint(IPAddress.Broadcast, remotePort);

Send(sendBytes, sendBytes.Length, remoteIpEnd);

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Finding Servers

Normally, servers operate on well-known ports These are ports which are less that 1024 and

are assigned for specific purposes Eg. Web servers are on port 80 This tells us what port the server is on What if we don’t know which machine is hosting

the server?

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Finding Servers

In this case, we can broadcast for a server Send out a broadcast message to all machines on the

LAN Wait for a reply Use that machine as a server

You can broadcast with UDP on one port to locate the servers

You can then stay with UDP to talk to the servers on another port or switch to TCP/IP on another port